ABSTRACT DNA replication is a fundamental process for all organisms to precisely duplicate genetic material prior to cell division. Central to the process is a helicase enzyme that utilizes ATP-hydrolysis to separate base- paired DNA to allow polymerases to gain access to synthesize complementary strands as well as to drive the replication machinery along the DNA. In human and other eukaryotic cells, the helicase engine is the hexameric MCM complex. The mechanisms that MCMs use to operate upon DNA are poorly understood at the molecular level. The proposed research will fill a knowledge gap by providing detailed pictures of MCM proteins interacting with DNA and ATP compounds. These will be studied at the molecular level by a coordinated approach involving structural studies by X-ray crystallography and in vitro methods to study their functions and interactions. A considerable body of preliminary data has been obtained for this project that includes MCM:DNA co-crystal structures, important preliminary diffracting crystals of MCM:ADP, purified proteins, and functional information from SPR studies.